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Abstract:

A tire, the tread of which comprises a rubber composition comprising at
least one diene elastomer, such as SBR and/or BR, a reinforcing filler,
such as silica and/or carbon black, and more than 10 phr, preferably more
than 15 phr, of a hydrogenated stirene thermoplastic elastomer. This
composition, which exhibits a reduced viscosity in the raw state, makes
it possible to obtain treads of tires exhibiting a reduced rolling
resistance, while retaining a good level of wet grip.

Claims:

1. A tire, the tread of which comprises a rubber composition comprising
at least one diene elastomer, one reinforcing filler and more than 10 phr
of a hydrogenated stirene thermoplastic ("TPS") elastomer.

2. The tire according to claim 1, wherein the diene elastomer is chosen
from the group consisting of polybutadienes (BR), synthetic polyisoprenes
(IR), natural rubber (NR), butadiene copolymers, isoprene copolymers and
the mixtures of these elastomers.

3. The tire according to claim 1, wherein the content of hydrogenated TPS
elastomer is greater than 15 phr.

4. The tire according to claim 3, wherein the content of hydrogenated TPS
elastomer is between 16 and 50 phr.

6. The tire according to claim 5, wherein the diene blocks are isoprene
or butadiene blocks.

7. The tire according to claim 6, wherein the unsaturated TPS copolymer
is chosen from the group consisting of stirene/butadiene (SB),
stirene/isoprene (SI), stirene/butadiene/butylene (SBB),
stirene/butadiene/isoprene (SBI), stirene/butadiene/stirene (SBS),
stirene/butadiene/butylene/stirene (SBBS), stirene/isoprene/stirene (SIS)
and stirene/butadiene/isoprene/stirene (SBIS) block copolymers and the
mixtures of these copolymers.

8. The tire according to claim 1, wherein the rubber composition
additionally comprises a plasticizing agent.

9. The tire according to claim 8, wherein the content of plasticizing
agent is between 5 and 50 phr.

10. The tire according to claim 8, wherein the plasticizing agent is a
thermoplastic hydrocarbon resin, the glass transition temperature (Tg) of
which is greater than 0.degree. C.

11. The tire according to claim 10, wherein the thermoplastic hydrocarbon
resin is chosen from the group consisting of cyclopentadiene (abbreviated
to CPD) or dicyclopentadiene (abbreviated to DCPD) homopolymer or
copolymer resins, terpene homopolymer or copolymer resins, terpene/phenol
homopolymer or copolymer resins, C5 fraction homopolymer or copolymer
resins, C9 fraction homopolymer or copolymer resins,
α-methylstirene homopolymer or copolymer resins, and the mixtures
of these resins.

12. The tire according to claim 8, wherein the plasticizing agent is
liquid at 20.degree. C. and has a glass transition temperature (Tg) of
less than -20.degree. C.

13. The tire according to claim 12, wherein the liquid plasticizing agent
is chosen from the group consisting of naphthenic oils, paraffinic oils,
MES oils, TDAE oils, ester plasticizers, ether plasticizers, phosphate
plasticizers, sulphonate plasticizers and the mixtures of these
compounds.

14. The tire according to claim 8, wherein the rubber composition
comprises a hydrocarbon resin, the glass transition temperature (Tg) of
which is greater than 0.degree. C. and a liquid plasticizing agent that
is liquid at 20.degree. C. and has a glass transition temperature (Tg) of
less than -20.degree. C.

15. The tire according to claim 1, wherein the reinforcing filler
comprises carbon black, silica or a mixture of carbon black and silica.

Description:

[0001] The present invention relates to tire treads and to rubber
compositions based on a diene elastomer and on a thermoplastic elastomer
which can be used in the manufacture of such tire treads.

[0002] In a known way, a tire has to meet a large number of often
conflicting technical requirements, including a high wear resistance, a
low rolling resistance and a high wet grip.

[0003] This compromise in properties, in particular from the viewpoint of
the rolling resistance and the grip, was able to be improved in recent
years with regard to energy-saving Green Tires, intended in particular
for passenger vehicles, by virtue in particular of the use of novel
weakly hysteretic rubber compositions having a characteristic of being
reinforced predominantly with specific inorganic fillers, described as
reinforcing, in particular of highly dispersible silicas, referred to as
"HDS" (Highly Dispersible Silica), capable of rivalling, from the
viewpoint of the reinforcing power, conventional tire-grade carbon
blacks.

[0004] The decrease in the rolling resistance of tire treads, without
damaging the other properties, in particular those of wet grip, remains,
however, a constant preoccupation of designers of tires.

[0005] During their research studies, the Applicant Companies have
discovered a rubber composition, based on a diene elastomer, on a
specific TPS elastomer and on a reinforcing filler, which makes it
possible to obtain tire treads exhibiting a reduced rolling resistance,
while retaining a good level of wet grip.

[0006] Thus, the invention relates to a tire, the tread of which comprises
a rubber composition comprising at least one diene elastomer, one
reinforcing filler and more than 10 phr of a hydrogenated stirene
thermoplastic elastomer.

[0008] The invention and its advantages will be easily understood in the
light of the description and implementation examples which follow.

I--MEASUREMENTS AND TESTS USED

[0009] The rubber compositions used in the tires according to the
invention are characterized, before and after curing, as indicated below.

[0010] I-1. Mooney Plasticity:

[0011] Use is made of an oscillating consistometer as described in French
Standard NF T 43-005 (1991). The Mooney plasticity measurement is carried
out according to the following principle: the composition in the raw
state (i.e., before curing) is moulded in a cylindrical chamber heated to
100° C. After preheating for one minute, the rotor rotates within
the test specimen at 2 revolutions/minute and the working torque for
maintaining this movement is measured after rotating for 4 minutes. The
Mooney plasticity (ML 1+4) is expressed in "Mooney unit" (MU, with 1
MU=0.83 newtonmetre).

[0012] I-2. Tensile Tests

[0013] These tensile tests make it possible to determine the elasticity
stresses and the properties at break. Unless otherwise indicated, they
are carried out in accordance with French Standard NF T 46-002 of
September 1988. The nominal secant moduli (or apparent stresses, in MPa)
are measured in second elongation (i.e., after a cycle of accommodation
to the degree of extension expected for the measurement itself) at 10%
elongation (denoted MA10) and at 100% elongation (denoted MA100). All
these tensile measurements are carried out under the standard conditions
of temperature (23+2° C.) and hygrometry (50±5% relative
humidity), according to French Standard NF T 40-101 (December 1979).

[0014] I-3. Shore A Hardness

[0015] The Shore A hardness of the compositions after curing is assessed
in accordance with Standard ASTM D 2240-86.

[0016] I-4. Dynamic Properties

[0017] The dynamic properties are measured on a viscosity analyser
(Metravib VA4000) according to Standard ASTM D 5992-96. The response of a
sample of vulcanized composition (cylindrical test specimen with a
thickness of 4 mm and with a cross section of 400 mm2), subjected to
a simple alternating sinusoidal shear stress, at a frequency of 10 Hz,
during a temperature sweep, under a stationary stress of 0.7 MPa, is
recorded; the tan(δ) value observed at 0° C. (i.e.,
tan(δ)0° C.) and the tan(δ) value observed at
40° C. (i.e., tan(δ)40° C.) are recorded.

[0018] It should be remembered that, in a way well known to a person
skilled in the art, the tan(δ)0° C. value is
representative of the wet grip potential: the higher
tan(δ)0° C., the better the grip. The
tan(δ)40° C. value is representative of the hysteresis
of the material and thus of the rolling resistance: the lower
tan(δ)40° C., the lower the rolling resistance.

II--CONDITIONS FOR THE IMPLEMENTATION OF THE INVENTION

[0019] The tread of the tire according to the invention comprises a rubber
composition comprising at least one diene elastomer, one reinforcing
filler and more than 10 phr of a hydrogenated stirene thermoplastic
("TPS") elastomer.

[0020] "phr" means parts by weight per hundred parts of total elastomer,
thus including the hydrogenated TPS elastomer.

[0021] In the present description, unless expressly indicated otherwise,
all the percentages (%) shown are % by weight. Moreover, any interval of
values denoted by the expression "between a and b" represents the range
of values extending from greater than a to less than b (i.e., limits a
and b excluded), whereas any interval of values denoted by the expression
"from a to b" means the range of values extending from a up to b (i.e.,
including the strict limits a and b).

[0022] II.1--Diene Elastomer

[0023] The tread of the tire according to the invention comprises a rubber
composition which has the essential first characteristic of comprising at
least one diene elastomer.

[0024] It should be remembered here that elastomer (or "rubber", the two
terms being regarded as synonyms) of the "diene" type should be
understood as meaning, in a known way, an (one or more are understood)
elastomer resulting at least in part (i.e., a homopolymer or a copolymer)
from diene monomers (monomers carrying two carbon-carbon double bonds,
which may or may not be conjugated).

[0025] The diene elastomers can be classified into two categories:
"essentially unsaturated" or "essentially saturated". The term
"essentially unsaturated" is understood to mean generally a diene
elastomer resulting at least in part from conjugated diene monomers
having a level of units of diene origin (conjugated dienes) which is
greater than 15% (molar %); thus it is that diene elastomers such as
butyl rubbers or copolymers of dienes and of α-olefins of EPDM type
do not come within the preceding definition and can in particular be
described as "essentially saturated" diene elastomers (low or very low
level of units of diene origin, always less than 15%). In the category of
"essentially unsaturated" diene elastomers, the term "highly unsaturated"
diene elastomer is understood to mean in particular a diene elastomer
having a level of units of diene origin (conjugated dienes) which is
greater than 50%.

[0026] Given these definitions, the term diene elastomer capable of being
used in the compositions in accordance with the invention is understood
more particularly to mean: [0027] (a)--any homopolymer obtained by
polymerization of a conjugated diene monomer having from 4 to 12 carbon
atoms; [0028] (b)--any copolymer obtained by copolymerization of one or
more conjugated dienes with one another or with one or more vinylaromatic
compounds having from 8 to 20 carbon atoms; [0029] (c)--a ternary
copolymer obtained by copolymerization of ethylene and of an
α-olefin having from 3 to 6 carbon atoms with a non-conjugated
diene monomer having from 6 to 12 carbon atoms, such as, for example, the
elastomers obtained from ethylene and propylene with a non-conjugated
diene monomer of the abovementioned type, such as, in particular,
1,4-hexadiene, ethylidenenorbornene or dicyclopentadiene; [0030] (d)--a
copolymer of isobutene and of isoprene (butyl rubber) and also the
halogenated versions, in particular chlorinated or brominated versions,
of this type of copolymer.

[0031] Although it applies to any type of diene elastomer, a person
skilled in the art of tires will understand that the present invention is
preferably employed with essentially unsaturated diene elastomers, in
particular of the type (a) or (b) above.

[0032] The following are suitable in particular as conjugated dienes:
1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C1-C5
alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,
2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or
2-methyl-3-isopropyl-1,3-butadiene, aryl-1,3-butadiene, 1,3-pentadiene or
2,4-hexadiene. The following, for example, are suitable as vinylaromatic
compounds: stirene, ortho-, meta- or para-methylstirene, the
"vinyltoluene" commercial mixture, para-(tert-butyl)stirene,
methoxystirenes, chlorostirenes, vinylmesitylene, divinylbenzene or
vinylnaphthalene.

[0033] The copolymers can comprise between 99% and 20% by weight of diene
units and between 1% and 80% by weight of vinylaromatic units. The
elastomers can have any microstructure which depends on the
polymerization conditions used, in particular on the presence or absence
of a modifying and/or randomizing agent and on the amounts of modifying
and/or randomizing agent employed. The elastomers can, for example, be
block, random, sequential or microsequential elastomers and can be
prepared in dispersion or in solution; they can be coupled and/or
star-branched or also functionalized with a coupling and/or
star-branching or functionalization agent. For coupling with carbon
black, mention may be made, for example, of functional groups comprising
a C--Sn bond or of aminated functional groups, such as aminobenzophenone,
for example; for coupling with a reinforcing inorganic filler, such as
silica, mention may be made, for example, of silanol functional groups or
polysiloxane functional groups having a silanol end (such as described,
for example, in FR 2 740 778, U.S. Pat. No. 6,013,718 and WO
2008/141702), of alkoxysilane groups (such as described, for example, in
FR 2 765 882 or U.S. Pat. No. 5,977,238), of carboxyl groups (such as
described, for example, in WO 01/92402 or U.S. Pat. No. 6,815,473, WO
2004/096865 or US 2006/0089445) or of polyether groups (such as
described, for example, in EP 1 127 909, U.S. Pat. No. 6,503,973, WO
2009/000750 and WO 2009/000752). Mention may also be made, as other
examples of functionalized elastomers, of elastomers (such as SBR, BR, NR
or IR) of the epoxidized type.

[0034] The following are suitable: polybutadienes, in particular those
having a content (molar %) of 1,2-units of between 4% and 80% or those
having a content (molar %) of cis-1,4-units of greater than 80%,
polyisoprenes, butadiene/stirene copolymers and in particular those
having a Tg (glass transition temperature, measured according to ASTM
D3418) of between 0° C. and -70° C. and more particularly
between -10° C. and -60° C., a stirene content of between
5% and 60% by weight and more particularly between 20% and 50%, a content
(molar %) of 1,2-bonds of the butadiene part of between 4% and 75% and a
content (molar %) of trans-1,4-bonds of between 10% and 80%,
butadiene/isoprene copolymers, in particular those having an isoprene
content of between 5% and 90% by weight and a Tg of -40° C. to
-80° C., or isoprene/stirene copolymers, in particular those
having a stirene content of between 5% and 50% by weight and a Tg of
between -5° C. and -50° C. In the case of
butadiene/stirene/isoprene copolymers, those having a stirene content of
between 5% and 50% by weight and more particularly of between 10% and
40%, an isoprene content of between 15% and 60% by weight and more
particularly between 20% and 50%, a butadiene content of between 5% and
50% by weight and more particularly of between 20% and 40%, a content
(molar %) of 1,2-units of the butadiene part of between 4% and 85%, a
content (molar %) of trans-1,4-units of the butadiene part of between 6%
and 80%, a content (molar %) of 1,2- plus 3,4-units of the isoprene part
of between 5% and 70% and a content (molar %) of trans-1,4-units of the
isoprene part of between 10% and 50%, and more generally any
butadiene/stirene/isoprene copolymer having a Tg of between -5° C.
and -70° C., are suitable in particular.

[0035] To sum up, the diene elastomer of the composition according to the
invention is preferably chosen from the group of the highly unsaturated
diene elastomers consisting of polybutadienes (abbreviated to "BR"),
synthetic polyisoprenes (IR), natural rubber (NR), butadiene copolymers,
isoprene copolymers and the mixtures of these elastomers. Such copolymers
are more preferably chosen from the group consisting of butadiene/stirene
copolymers (SBR), isoprene/butadiene copolymers (BIR), isoprene/stirene
copolymers (SIR) and isoprene/butadiene/stirene copolymers (SBIR).

[0036] According to a specific embodiment, the diene elastomer is
predominantly (i.e., for more than 50 phr) an SBR alone, whether an SBR
prepared in emulsion ("ESBR") or an SBR prepared in solution ("SSBR").

[0037] According to another specific embodiment, the diene elastomer is an
SBR/BR blend (mixture).

[0038] According to other possible embodiments, the diene elastomer is an
SBR/NR (or SBR/IR), BR/NR (or BR/IR) or SBR/BR/NR (or SBR/BR/IR) blend.

[0039] In the case of an SBR (ESBR or SSBR) elastomer, use is made in
particular of an SBR having a moderate stirene content, for example of
between 20% and 35% by weight, or a high stirene content, for example
from 35% to 45%, a content of vinyl bonds of the butadiene part of
between 15% and 70%, a content (molar %) of trans-1,4-bonds of between
15% and 75% and a Tg of between -10° C. and -55° C.; such
an SBR can advantageously be used as a mixture with a BR preferably
having more than 90% (molar %) of cis-1,4-bonds.

[0040] According to another specific embodiment, the diene elastomer is an
isoprene elastomer. The term "isoprene elastomer" is understood to mean,
in a known way, an isoprene homopolymer or copolymer, in other words a
diene elastomer chosen from the group consisting of natural rubber (NR),
which may be plasticized or peptized, synthetic polyisoprenes (IR), the
various copolymers of isoprene and the mixtures of these elastomers.
Mention will in particular be made, among isoprene copolymers, of
isobutene/isoprene copolymers (butyl rubber--IIR), isoprene/stirene
copolymers (SIR), isoprene/butadiene copolymers (BIR) or
isoprene/butadiene/stirene copolymers (SBIR). This isoprene elastomer is
preferably natural rubber or a synthetic cis-1,4-polyisoprene; use is
preferably made, among these synthetic polyisoprenes, of the
polyisoprenes having a level (molar %) of cis-1,4-bonds of greater than
90%, more preferably still of greater than 98%.

[0041] According to another preferred embodiment of the invention, the
rubber composition comprises a blend of a (one or more) "high Tg" diene
elastomer exhibiting a Tg of between -70° C. and 0° C. and
of a (one or more) "low Tg" diene elastomer exhibiting a Tg of between
-110° C. and -80° C., more preferably between -105°
C. and -90° C. The high Tg elastomer is preferably chosen from the
group consisting of S-SBRs, E-SBRs, natural rubber, synthetic
polyisoprenes (exhibiting a level (molar %) of cis-1,4-structures
preferably of greater than 95%), BIRs, SIRs, SBIRs and the mixtures of
these elastomers. The low Tg elastomer preferably comprises butadiene
units according to a level (molar %) at least equal to 70%; it preferably
consists of a polybutadiene (BR) exhibiting a level (molar %) of
cis-1,4-structures of greater than 90%.

[0042] According to another specific embodiment of the invention, the
rubber composition comprises, for example, between 30 and 90 phr, in
particular between 40 and 90 phr, of a high Tg elastomer as a blend with
a low Tg elastomer.

[0043] According to another specific embodiment of the invention, the
diene elastomer of the composition according to the invention comprises a
blend of a BR (as low Tg elastomer) exhibiting a level (molar %) of
cis-1,4-structures of greater than 90% with one or more S-SBRs or E-SBRs
(as high Tg elastomer(s)).

[0044] The compositions according to the invention can comprise a single
diene elastomer or a mixture of several diene elastomers.

[0045] II.2--Hydrogenated TPS Elastomer

[0046] The tread of the tire according to the invention comprises a rubber
composition which has the other essential characteristic of comprising
more than 10 phr of a hydrogenated stirene thermoplastic elastomer.

[0047] It should be remembered first of all that stirene thermoplastic
elastomers (also called "TPS" elastomers) are thermoplastic elastomers
existing in the form of stirene-based block copolymers. Of intermediate
structure between thermoplastic polymers and elastomers, they are
composed, in a known way, of rigid polystirene sequences connected by
flexible elastomer sequences, for example polybutadiene, polyisoprene or
poly(ethylene/butylene). They are often triblock elastomers with two
rigid segments connected by a flexible segment. The rigid and flexible
segments can be positioned linearly, in star fashion or in branched
fashion. These TPS elastomers can also be diblock elastomers with just
one rigid segment connected to a flexible segment. Typically, each of
these segments or blocks comprises a minimum of more than 5, generally
more than 10, base units (for example, stirene units and isoprene units
for a stirene/isoprene/stirene block copolymer).

[0048] It should also be remembered that unsaturated TPS copolymer should
be understood as meaning a TPS copolymer which is provided with ethylenic
unsaturations, that is to say comprises carbon-carbon double bonds
(conjugated or nonconjugated). Saturated TPS copolymer is understood to
mean a TPS copolymer which does not comprise any ethylenic unsaturation
(i.e., any carbon-carbon double bond).

[0049] In a known way and by definition in the present patent application,
a "hydrogenated" TPS elastomer is an elastomer which is partially or
completely hydrogenated, that is to say obtained by partial or complete
hydrogenation of an unsaturated TPS elastomer.

[0050] According to a preferred embodiment, the hydrogenated TPS elastomer
results from an unsaturated TPS copolymer comprising stirene blocks and
diene blocks, these diene blocks being in particular isoprene or
butadiene blocks.

[0051] According to another preferred embodiment, the above TPS copolymer
is chosen from the group consisting of stirene/butadiene (SB),
stirene/isoprene (SI), stirene/butadiene/butylene (SBB),
stirene/butadiene/isoprene (SBI), stirene/butadiene/stirene (SBS),
stirene/butadiene/butylene/stirene (SBBS), stirene/isoprene/stirene (SIS)
and stirene/butadiene/isoprene/stirene (SBIS) block copolymers and the
mixtures of these copolymers.

[0052] According to a more preferred embodiment, the hydrogenated TPS
results from an unsaturated TPS copolymer which comprises
stirene/isoprene/stirene (SIS) blocks and can comprise, for example,
butadiene blocks.

[0053] According to another preferred embodiment of the invention, the
stirene content of the TPS elastomer is between 5 and 50%. Below the
minimum indicated, there is a risk of the thermoplastic nature of the
elastomer being substantially reduced whereas, above the recommended
maximum, the elasticity of the composition may be affected. For these
reasons, the stirene content is more preferably between 10 and 40%.

[0054] The number-average molecular weight (denoted Mn) of the TPS
elastomer is preferably between 50 000 and 500 000 g/mol, more preferably
between 75 000 and 450 000. This weight is determined in a known way, by
steric exclusion chromatography (SEC). The sample is dissolved beforehand
in tetrahydrofuran at a concentration of approximately 1 g/l; the
solution is then filtered through a filter with a porosity of 0.45 μm
before injection. The equipment used is a "Waters Alliance"
chromatographic line. The elution solvent is tetrahydrofuran, the flow
rate is 0.7 ml/min, the temperature of the system is 35° C. and
the analytical time is 90 min. A set of four Waters columns in series,
with the "Styragel" trade names ("HMW7", "HMW6E" and two "HT6E"), is
used. The injected volume of the solution of the polymer sample is 100
μl. The detector is a "Waters 2410" differential refractometer and its
associated software for making use of the chromatographic data is the
"Waters Millennium" system. The calculated average molar masses are
relative to a calibration curve produced with polystirene standards.

[0055] The rubber composition preferably comprises more than 15 phr of
hydrogenated TPS elastomer, preferably between 16 and 50 phr, for example
from 18 to 40 phr, of such an elastomer. Below the minima indicated,
there is a risk of the technical effect obtained being insufficient;
above the maxima indicated, there is a risk of the grip being
detrimentally affected.

[0056] Hydrogenated TPS elastomers resulting from unsaturated TPSs, such
as those mentioned above, are well known and are commercially available,
for example are sold by Kuraray under the "Hybrar" name of the 7000
series.

[0057] II.3--Reinforcing Filler

[0058] Use may be made of any type of reinforcing filler known for its
capabilities of reinforcing a rubber composition which can be used for
the manufacture of tires, for example an organic filler, such as carbon
black, a reinforcing inorganic filler, such as silica, or a mixture of
these two types of filler, in particular a mixture of carbon black and
silica.

[0059] All carbon blacks, in particular "tire-grade" blacks, are suitable
as carbon blacks. Mention will more particularly be made, among the
latter, of the reinforcing carbon blacks of the 100, 200 or 300 series
(ASTM grades), such as, for example, the N115, N134, N234, N326, N330,
N339, N347 or N375 blacks, or also, depending on the applications
targeted, the blacks of higher series (for example, N660, N683 or N772).
The carbon blacks might, for example, be already incorporated in an
isoprene elastomer in the form of a masterbatch (see, for example,
Applications WO 97/36724 or WO 99/16600).

[0060] Mention may be made, as examples of organic fillers other than
carbon blacks, of the functionalized polyvinylaromatic organic fillers as
described in Applications WO-A-2006/069792, WO-A-2006/069793,
WO-A-2008/003434 and WO-A-2008/003435.

[0061] The term "reinforcing inorganic filler" should be understood, in
the present patent application, by definition, as meaning any inorganic
or mineral filler (whatever its colour and its origin, natural or
synthetic), also known as "white filler", "clear filler" or even
"non-black filler", in contrast to carbon black, capable of reinforcing
by itself alone, without means other than an intermediate coupling agent,
a rubber composition intended for the manufacture of tires, in other
words capable of replacing, in its reinforcing role, a conventional
tire-grade carbon black; such a filler is generally characterized, in a
known way, by the presence of hydroxyl (--OH) groups at its surface.

[0062] The physical state under which the reinforcing inorganic filler is
provided is not important, whether it is in the form of a powder, of
microbeads, of granules, of beads or any other appropriate densified
form. Of course, the term reinforcing inorganic filler is also understood
to mean mixtures of different reinforcing inorganic fillers, in
particular of highly dispersible siliceous and/or aluminous fillers as
described below.

[0063] Mineral fillers of the siliceous type, in particular silica
(SiO2), or of the aluminous type, in particular alumina
(Al2O3), are suitable in particular as reinforcing inorganic
fillers. The silica used can be any reinforcing silica known to a person
skilled in the art, in particular any precipitated or pyrogenic silica
exhibiting a BET surface and a CTAB specific surface both of less than
450 m2/g, preferably from 30 to 400 m2/g. Mention will be made,
as highly dispersible ("HDS") precipitated silicas, for example, of the
Ultrasil 7000 and Ultrasil 7005 silicas from Degussa, the Zeosil 1165 MP,
1135 MP and 1115 MP silicas from Rhodia, the Hi-Sil EZ150G silica from
PPG, the Zeopol 8715, 8745 and 8755 silicas from Huber or the silicas
with a high specific surface as described in Application WO 03/16837.

[0064] The reinforcing inorganic filler used, in particular if it is
silica, preferably has a BET specific surface of between 45 and 400
m2/g, more preferably of between 60 and 300 m2/g.

[0065] Preferably, the content of total reinforcing filler (carbon black
and/or reinforcing inorganic filler, such as silica) is between 20 and
200 phr, more preferably between 30 and 150 phr, the optimum being in a
known way different depending on the specific applications targeted: the
level of reinforcement expected with regard to a bicycle tire, for
example, is, of course, less than that required with regard to a tire
capable of running at high speed in a sustained manner, for example a
motorcycle tire, a tire for a passenger vehicle or a tire for a utility
vehicle, such as a heavy-duty vehicle.

[0066] According to a preferred embodiment of the invention, use is made
of a reinforcing filler comprising between 30 and 150 phr, more
preferably between 50 and 120 phr, of inorganic filler, particularly
silica, and optionally carbon black; the carbon black, when it is
present, is preferably used at a content of less than 20 phr, more
preferably of less than 10 phr (for example, between 0.1 and 10 phr).

[0067] In order to couple the reinforcing inorganic filler to the diene
elastomer, use is made, in a known way, of an at least bifunctional
coupling agent (or bonding agent) intended to provide a satisfactory
connection, of chemical and/or physical nature, between the inorganic
filler (surface of its particles) and the diene elastomer, in particular
bifunctional organosilanes or polyorganosiloxanes. Use is made in
particular of silane polysulphides, referred to as "symmetrical" or
"unsymmetrical" depending on their specific structure, as described, for
example, in Applications WO 03/002648 (or US 2005/016651) and WO
03/002649 (or US 2005/016650).

[0068] "Symmetrical" silane polysulphides corresponding to the following
general formula (I):

Z-A-Sx-A-Z,

in which: [0069] x is an integer from 2 to 8 (preferably from 2 to 5);
[0070] A is a divalent hydrocarbon radical (preferably, C1-C18
alkylene groups or C6-C12 arylene groups, more particularly
C1-C10, in particular C1-C4 alkylenes, especially
propylene); [0071] Z corresponds to one of the formulae below:

[0071] ##STR00001## [0072] in which: [0073] the R1 radicals,
which are unsubstituted or substituted and identical to or different from
one another, represent a C1-C18 alkyl, C5-C18
cycloalkyl or C6-C18 aryl group (preferably, C1-C6
alkyl, cyclohexyl or phenyl groups, in particular C1-C4 alkyl
groups, more particularly methyl and/or ethyl). [0074] the R2
radicals, which are unsubstituted or substituted and identical to or
different from one another, represent a C1-C18 alkoxyl or
C5-C18 cycloalkoxyl group (preferably a group chosen from
C1-C8 alkoxyls and C5-C8 cycloalkoxyls, more
preferably still a [0075] group chosen from C1-C4 alkoxyls,
in particular methoxyl and ethoxyl), are suitable in particular, without
the above definition being limiting.

[0076] In the case of a mixture of alkoxysilane polysulphides
corresponding to the above formula (I), in particular the usual mixtures
available commercially, the mean value of the "x" index is a fractional
number preferably of between 2 and 5, more preferably in the vicinity of
4. However, the invention can also advantageously be carried out, for
example, with alkoxysilane disulphides (x=2).

[0077] Mention will more particularly be made, as examples of silane
polysulphides, of
bis((C1-C4)alkoxyl(C1-C4)alkylsilyl(C1-C4)a-
lkyl)polysulphides (in particular disulphides, trisulphides or
tetrasulphides), such as, for example, bis(3-trimethoxysilylpropyl) or
bis(3-triethoxysilylpropyl)polysulphides. Use is in particular made,
among these compounds, of bis(3-triethoxysilylpropyl)tetrasulphide,
abbreviated to TESPT, of formula
[(C2H5O)3Si(CH2)3S2]2, or
bis(3-triethoxysilylpropyl)disulphide, abbreviated to TESPD, of formula
[(C2H5O)3Si(CH2)3S]2. Mention will also be
made, as preferred examples, of
bis(mono(C1-C4)alkoxyldi(C1-C4)alkylsilylpropyl)polys-
ulphides (in particular disulphides, trisulphides or tetrasulphides), more
particularly bis(monoethoxydimethylsilylpropyl)tetrasulphide, as
described in Patent Application WO 02/083782 (or US 2004/132880).

[0078] Mention will in particular be made, as coupling agent other than
alkoxysilane polysulphide, of bifunctional POSs (polyorganosiloxanes) or
of hydroxysilane polysulphides (R2═OH in the above formula I),
such as described in Patent Applications WO 02/30939 (or U.S. Pat. No.
6,774,255) and WO 02/31041 (or US 2004/051210), or of silanes or POSs
carrying azodicarbonyl functional groups, such as described, for example,
in Patent Applications WO 2006/125532, WO 2006/125533 and WO 2006/125534.

[0079] In the rubber compositions in accordance with the invention, the
content of coupling agent is preferably between 4 and 12 phr, more
preferably between 4 and 8 phr.

[0080] A person skilled in the art will understand that a reinforcing
filler of another nature, in particular organic nature, might be used as
filler equivalent to the reinforcing inorganic filler described in the
present section, provided that this reinforcing filler is covered with an
inorganic layer, such as silica, or else comprises, at its surface,
functional sites, in particular hydroxyls, requiring the use of a
coupling agent in order to form the connection between the filler and the
elastomer.

[0081] II.4--Various Additives

[0082] The rubber compositions of the treads of the tires in accordance
with the invention also comprise all or a portion of the usual additives
generally used in elastomer compositions intended for the manufacture of
treads, such as, for example, pigments, protection agents, such as
antiozone waxes, chemical antiozonants, antioxidants, plasticizing agents
other than those mentioned above, antifatigue agents, reinforcing resins,
methylene acceptors (for example, phenolic novolac resin) or methylene
donors (for example, HMT or H3M), a crosslinking system based either on
sulphur or on sulphur donors and/or on peroxide and/or on bismaleimides,
vulcanization accelerators or vulcanization activators.

[0083] These compositions can also comprise, in addition to coupling
agents, coupling activators, agents for covering the inorganic fillers or
more generally processing aids capable, in a known way, by virtue of an
improvement in the dispersion of the filler in the rubber matrix and of a
lowering in the viscosity of the compositions, of improving their
processing property in the raw state, these agents being, for example,
hydrolysable silanes, such as alkylalkoxysilanes, polyols, polyethers,
primary, secondary or tertiary amines, or hydroxylated or hydrolysable
polyorganosiloxanes.

[0084] According to a preferred embodiment, the composition according to
the invention additionally comprises a plasticizing agent. Preferably,
this plasticizing agent is a solid hydrocarbon resin, a liquid
plasticizer or a mixture of the two.

[0085] The content of plasticizing agent is preferably between 5 and 50
phr, more preferably between 10 and 40 phr, for example between 15 and 35
phr.

[0086] According to a first preferred embodiment of the invention, this
plasticizing agent is a hydrocarbon resin, the Tg of which is greater
than 0° C., preferably greater than +20° C.

[0087] In a way known to a person skilled in the art, the designation
"resin" is reserved, in the present patent application, by definition,
for a thermoplastic compound which is a solid at ambient temperature
(23° C.), in contrast to a liquid plasticizing compound, such as
an oil.

[0088] Preferably, the thermoplastic hydrocarbon plasticizing resin
exhibits at least any one of the following characteristics: [0089] a Tg
of greater than 20° C., more preferably of greater than 30°
C.; [0090] a number-average molecular weight (Mn) of between 400 and 2000
g/mol, more preferably between 500 and 1500 g/mol; [0091] a
polydispersity index (PI) of less than 3, more preferably of less than 2
(reminder: PI=Mw/Mn, with Mw the weight-average molecular weight).

[0092] More preferably, this hydrocarbon plasticizing resin exhibits all
of the preferred characteristics above.

[0094] The hydrocarbon resins can be aliphatic or aromatic or of the
aliphatic/aromatic type, that is to say based on aliphatic and/or
aromatic monomers. They can be natural or synthetic, based or not based
on oil (if such is the case, also known under the name of petroleum
resins). Stirene, α-methylstirene, ortho-, meta- or
para-methylstirene, vinyltoluene, para-(tert-butyl)stirene,
methoxystirenes, chlorostirenes, vinylmesitylene, divinylbenzene,
vinylnaphthalene and any vinylaromatic monomer resulting from a C9
fraction (or more generally from a C8 to C10 fraction), for
example, are suitable as aromatic monomers. Preferably, the vinylaromatic
monomer is stirene or a vinylaromatic monomer resulting from a C9
fraction (or more generally from a C8 a C10 fraction).
Preferably, the vinylaromatic monomer is the minor monomer, expressed as
molar fraction, in the copolymer under consideration.

[0095] According to a particularly preferred embodiment, the plasticizing
hydrocarbon resin is chosen from the group consisting of cyclopentadiene
(abbreviated to CPD) or dicyclopentadiene (abbreviated to DCPD)
homopolymer or copolymer resins, terpene homopolymer or copolymer resins,
terpene/phenol homopolymer or copolymer resins, C5 fraction
homopolymer or copolymer resins, C9 fraction homopolymer or
copolymer resins, α-methylstirene homopolymer or copolymer resins,
and the mixtures of these resins, which can be used alone or in
combination with a liquid plasticizer, for example an MES or TDAE oil.

[0096] The term "terpene" combines here, in a known way, the
α-pinene, β-pinene and limonene monomers; use is preferably
made of a limonene monomer, which compound exists, in a known way, in the
form of three possible isomers: L-limonene (laevorotatory enantiomer),
D-limonene (dextrorotatory enantiomer) or else dipentene, the racemate of
the dextrorotatory and laevorotatory enantiomers. Mention will in
particular be made, among the above plasticizing hydrocarbon resins, of
α-pinene, β-pinene or dipentene homo- or copolymer resins or
polylimonene resins.

[0097] The preferred resins above are well known to a person skilled in
the art and are commercially available, for example sold, as regards the:
[0098] polylimonene resins: by DRT under the name "Dercolyte L120"
(Mn=625 g/mol; Mw=1010 g/mol; PI=1.6; Tg=72° C.) or by Arizona
Chemical Company under the name "Sylvagum TR7125C" (Mn=630 g/mol; Mw=950
g/mol; PI=1.5; Tg=70° C.); [0099] C5 fraction/vinylaromatic
copolymer resins, in particular C5 fraction/stirene or C5
fraction/C9 fraction copolymer resin: by Neville Chemical Company
under the names "Super Nevtac 78", "Super Nevtac 85" or "Super Nevtac
99", by Goodyear Chemicals under the name "Wingtack Extra", by Kolon
under the names "Hikorez T1095" and "Hikorez T1100", of Exxon under the
names "Escorez 2101" and "ECR 373"; [0100] limonene/stirene copolymer
resins: by DRT under the name "Dercolyte TS 105" or by Arizona Chemical
Company under the names "ZT115LT" and "ZT5100".

[0101] Mention may also be made, as examples of other preferred resins, of
phenol-modified α-methylstirene resins. It should be remembered
that, in order to characterize these phenol-modified resins, use is made,
in a known way, of a number referred to as "hydroxyl number" (measured
according to Standard ISO 4326 and expressed in mg KOH/g).
α-Methylstirene resins, in particular those modified with phenol,
are well known to a person skilled in the art and are available
commercially, for example sold by Arizona Chemical Company under the
names "Sylvares SA 100" (Mn=660 g/mol; PI=1.5; Tg=53° C.);
"Sylvares SA 120" (Mn=1030 g/mol; PI=1.9; Tg=64° C.); "Sylvares
540" (Mn=620 g/mol; PI=1.3; Tg=36° C.; hydroxyl number=56 mg
KOH/g); and "Sylvares 600" (Mn=850 g/mol; PI=1.4; Tg=50° C.;
hydroxyl number=31 mg KOH/g).

[0102] According to another preferred embodiment of the invention, the
plasticizer is a plasticizer which is liquid at 20° C., referred
to as a "low Tg" plasticizer, that is to say which, by definition,
exhibits a Tg of less than -20° C., preferably of less than
-40° C.

[0103] Any extending oil, whether it is aromatic or nonaromatic in nature,
any liquid plasticizing agent known for its plasticizing properties with
regard to diene elastomers, can be used. Suitable in particular are the
liquid plasticizers chosen from the group consisting of naphthenic oils,
in particular hydrogenated naphthenic oils, paraffinic oils, MES oils,
TDAE oils, ester or ether plasticizers, phosphate and sulphonate
plasticizers, and the mixtures of these compounds.

[0104] Use may be made in particular, among ester plasticizers, of the
compounds chosen from the group consisting of the phosphates,
trimellitates, pyromellitates, phthalates,
1,2-cyclohexane-dicarboxylates, adipates, azelates, sebacates, glycerol
triesters and the mixtures of these compounds.

[0105] II.5--Preparation of the Rubber Compositions

[0106] The compositions used in the treads of the tires of the invention
can be manufactured in appropriate mixers using two successive
preparation phases well known to a person skilled in the art: a first
phase of thermomechanical working or kneading ("non-productive" phase) at
high temperature, up to a maximum temperature of between 110° C.
and 190° C., preferably between 130° C. and 180° C.,
followed by a second phase of mechanical working ("productive" phase), up
to a lower temperature, typically of less than 110° C., for
example between 40° C. and 100° C., finishing phase during
which the crosslinking system is incorporated.

[0107] The process for preparing such compositions comprises, for example,
the following stages: [0108] incorporating, in a diene elastomer,
during a first stage ("non-productive" stage), at least one reinforcing
filler and more than 10 phr of a hydrogenated TPS elastomer, everything
being kneaded thermomechanically (for example in one or more goes), until
a maximum temperature of between 110° C. and 190° C. is
reached; [0109] cooling the combined mixture to a temperature of less
than 100° C.; [0110] subsequently incorporating, during a second
stage ("productive" stage), a crosslinking system; [0111] kneading
everything up to a maximum temperature of less than 110° C.

[0112] By way of example, the non-productive phase is carried out in a
single thermomechanical stage during which, in a first step, all the
necessary base constituents (the diene elastomer, more than 10 phr of
hydrogenated TPS elastomer, the reinforcing filler) are introduced into
an appropriate mixer, such as a normal internal mixer, followed, in a
second step, for example after kneading for one to two minutes, by the
other additives, optional additional covering agents for the filler or
processing aids, with the exception of the crosslinking system. The total
duration of the kneading, in this non-productive phase, is preferably
between 1 and 15 min.

[0113] After cooling the mixture thus obtained, the crosslinking system is
then incorporated in an external mixer, such as an open mill, maintained
at low temperature (for example between 40° C. and 100°
C.). The combined mixture is then mixed (productive phase) for a few
minutes, for example between 2 and 15 min.

[0114] The crosslinking system proper is preferably based on sulphur and
on a primary vulcanization accelerator, in particular on an accelerator
of the sulphonamide type. Additional to this vulcanization system are
various known vulcanization activators or secondary vulcanization
accelerators, such as zinc oxide, stearic acid, guanidine derivatives (in
particular diphenylguanidine), and the like, incorporated during the
first non-productive phase and/or during the productive phase. The
sulphur content is preferably between 0.5 and 3.0 phr and the content of
the primary accelerator is preferably between 0.5 and 5.0 phr.

[0115] Use may be made, as (primary or secondary) accelerator, of any
compound capable of acting as accelerator for the vulcanization of diene
elastomers in the presence of sulphur, in particular accelerators of the
thiazole type and their derivatives or accelerators of thiuram or zinc
dithiocarbamate type. These accelerators are more preferably chosen from
the group consisting of 2-mercaptobenzothiazyl disulphide (abbreviated to
"MBTS"), N-cyclohexyl-2-benzothiazolesulphenamide (abbreviated to "CBS"),
N,N-dicyclohexyl-2-benzothiazole-sulphenamide (abbreviated to "DCBS"),
N-tert-butyl-2-benzothiazolesulphenamide (abbreviated to "TBBS"),
N-tert-butyl-2-benzothiazolesulphenimide (abbreviated to "TBSI"), zinc
dibenzyldithiocarbamate (abbreviated to "ZBEC") and the mixtures of these
compounds. Preferably, a primary accelerator of the sulphonamide type is
used.

[0116] The final composition thus obtained can subsequently be calendered,
for example in the form of a sheet or of a plaque, in particular for
characterization in the laboratory, or else extruded, for example to form
a rubber profiled element used in the manufacture of a tread.

[0117] The invention relates to the tires described above both in the raw
state (that is to say, before curing) and in the cured state (that is to
say, after crosslinking or vulcanization).

III--EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION

[0118] III.1--Preparation of the Compositions

[0119] The procedure for the tests which follow is as follows: the diene
elastomer, the hydrogenated TPS elastomer, the reinforcing filler (silica
and/or carbon black) and the various other ingredients, with the
exception of the vulcanization system, are successively introduced into
an internal mixer (final degree of filling: approximately 70% by volume),
the starting vessel temperature of which is approximately 60° C.
Thermomechanical working is then carried out (non-productive phase) in
one stage, which lasts in total approximately from 3 to 4 min, until a
maximum "dropping" temperature of 165° C. is reached.

[0120] The mixture thus obtained is recovered and cooled, and then sulphur
and an accelerator of sulphenamide type are incorporated on a mixer
(homofinisher) at 30° C., everything being mixed (productive
phase) for an appropriate time (for example between 5 and 12 min).

[0121] The compositions thus obtained are subsequently calendered, either
in the form of plaques (thickness of 2 to 3 mm) or of thin sheets of
rubber, for the measurement of their physical or mechanical properties,
or extruded in the form of a tread.

III.2--Tests

[0122] The tests which follow demonstrate the improvement, that is to say
the decrease in the rolling resistance of treads of the tires according
to the invention, in comparison with a control tread, without damaging
the grip.

[0123] For this, two rubber compositions for a tread were prepared as
indicated above, one in accordance with the invention (denoted
hereinafter C.2) and one not in accordance with the invention (control
composition, denoted hereinafter C.1).

[0124] Their formulations (in phr or parts by weight per hundred parts of
total elastomer, thus including the hydrogenated TPS elastomer) are
presented in the appended Table 1.

[0125] Composition C.1 is a reference composition for a person skilled in
the art, based on SBR and BR, which is conventionally used to manufacture
Green Tire treads for passenger vehicles.

[0126] Composition C.2 is based on SBR and BR and on a hydrogenated TPS
elastomer. This composition C.2 thus differs from the control composition
C.1 only in the replacement of phr of SBR by 20 phr of hydrogenated TPS
elastomer.

[0127] Compositions C.1 and C.2 both comprise a plasticizing agent
comprising a mixture of a hydrocarbon resin (polylimonene resin) and of a
liquid plasticizer (MES oil).

[0128] The properties of the compositions, before and after curing
(vulcanization), have been summarized in the appended Table 2.

[0129] It is noted first of all that composition C.2 exhibits a Mooney
viscosity value which is substantially lower than that of the control
composition C.1, which testifies to an improvement in the processability
of the compositions in the raw state.

[0130] Moreover, it is noted that compositions C.1 and C.2 exhibit, after
curing, equivalent properties of stiffness (Shore A hardness) and of
modulus at 10% strain and at 100% strain.

[0131] Finally, it is noted that composition C.2 of the tire according to
the invention exhibits: [0132] on the one hand, a
tan(δ)0° C. value identical to that of the control
composition C.1, which illustrates a wet grip potential identical to that
of the control composition C.1; [0133] on the other hand and in
particular, a tan(δ)40° C. value which is lower than
that of the control composition C.1, which is synonymous to a person
skilled in the art with a decrease in the hysteresis of the material and
thus in the rolling resistance of the tire.

[0134] To sum up, the combined use of a diene elastomer, such as a
stirene/butadiene copolymer or a polybutadiene, and of a hydrogenated TPS
elastomer at the recommended content makes it possible, on the one hand,
to reduce the viscosity of the compositions in the raw state, thus
promoting their processability, and, on the other hand, to reduce the
rolling resistance of these compositions used as treads for tires,
without damaging the wet grip.